Audie Leon Murphy, the most decorated American soldier ever. Awarded every medal that the United States Military can bestow upon a soldier. Read his Medal of Honor, it don't get any tougher than this.
Fedor Emelianenko...He once arm bared a grizzly bear because it looked at him wrong.
This was his toughest challenge to date, since he was too short to arm bar him or RNC he just had to KO him.
Alright, I don't know if he is the "toughest," but he is without a shadow of a doubt the badest man to ever walk the face of this earth.
Not entirely true...
Don't forget Sargent Alvin York.